This is the continuation of a long-term research project designed to gain fundamental information about the mechanisms of action of purine and pyrimidine analogues and about the enzymes with which these analogues or their derivatives interact. Studies on the pathways of polyphosphate nucleotide synthesis and degradation. Under study are the enzymes ATP: GMP phosphotransferase, nucleoside diphosphokinase, and adenosine deaminase. Marked heterogeneity has been observed with the first two enzymes in a variety of tissues; much attention is being payed to the isozymes of NDP kinase. Attempts are being made to isolate significant amounts of pure isozymes so that comparative studies of the enzymatic behavior and physical and chemical properties may be performed. Studies of the mechanism of cytolytic action of purine and purine ribonucleoside analogues. Considerable attention is being payed to the marked anti-cancer synergism between 6-methylthioinosine and thiopurines such as 6-thioguanine and 6-mercaptopurine. Possible mechanisms of this synergism have been identified with Sarcoma 180 cells and it is hoped to expand these studies to malignant tissue from humans. Studies with human erythrocytes as a source of human enzymes and as model cell for the study of purine nucleotide metabolism. A number of the enzymes under study are isolated from human erythrocytes, including NDP kinase, guanylate kinase and adenosine deaminase. In addition, the nucleotide metabolism of intact erythrocytes is under study. The ability of these cells to take up and incorporate into nucleotide pools natural purines and purine nucleosides as well as a number of purine analogs is under study.